Naturally self-centring

by Simon Hargeaves

Why balancing a bike feels like the most natural thing in the world...

Most of us have an innate feel for balancing a high centre of gravity on a relatively short wheelbase. Say, like a motorbike. Its centre of gravity is the point at which the sum of the surrounding mass is zero, and is largely determined by the location of the engine, the heaviest part of the package. Usually the CoG is midway between the wheels (half the wheelbase) and 60 to 75cm off the ground; just above the engine, in front of your knees.

Or at least it is until you get on. Your personal CoG moves the overall CoG back behind the fuel tank, and raises it slightly. Add luggage and/or pillion, and it shifts even higher and further to the rear. That's important in determining overall handling characteristics, but especially braking and acceleration, because it dictates two behaviours: the bike's weight distribution longitudinally – the balance of weight front-to-rear – and the height of the point through which the bike's inertia acts.

Weight balance is normally around 50/50 front to rear (more to the front for sportsbikes, to the rear for tourers). In the 60s and 70s manufacturers took a while to fix this position as they juggled with poor tyres, flexible frames and inferior suspension. It was commonplace to bias the CoG heavily to the rear, sometimes due to engine shape (long Ducati L-twins) and sometimes because they thought it was the only way for powerful engines to find traction. To maintain front stability and grip, steering geometry was raked out resulting in slow, heavy steering. However, modern bikes have an abundance of grip front and rear, and suspension and brakes that can utilise it.

When we hit the front brakes a force acts in opposition to the inertia of the bike and rider, which tries to keep moving forwards. Weight is pitched onto the front wheel, compressing fork springs and making the rear of the bike go light. We can feel this through our arms; if we don't resist we'll be thrown over the front. And if we have too much front grip, and/or the combined centre of gravity of the bike and rider is too high, and/or the wheelbase is too short, the bike will rotate around the front wheel and overturn (the bike's inertia acts through its centre of gravity while the brake force is at ground level, creating an over-turning force).

A similar situation exists under acceleration: when we open the throttle the front goes light and weight is transferred to the rear. We slide back and hang on and, again, if there's grip, a high CoG and a short wheelbase, the bike will wheelie over backwards.

Which is why pillions and luggage increase the propensity for your bike to go light at the front, or wheelie, under acceleration. By shifting the CoG to the rear, the load has increased on the rear tyre (it's got more traction), but there's an equivalent loss of load on the front tyre (the total load always remains the sum of you, your payload and the bike). If you actually pull a wheelie, your entire combined weight is loaded on the back tyre.

But it's rare for riders to flip a bike accidentally in either direction, because it turns out bikes aren't the only things with a short wheelbase and a high CoG. We have too: standing upright, our CoG is just above our hips, but our 'wheelbase', or footprint, is incredibly short: usually around a size 10. Just like a motorbike. Which explains why we find coping with bike dynamics so intuitive.

I wonder if Simon is reading these comments and wondering how the train of though got to doughnuts quite so quickly.

I dunno.

But if we happened upon a gigantic donut shaped planet, one with enough mass to generate gravity akin to that on earth, would we be pulled towards the dimensionless centre point or towards the centre of the annular portion?

Could I walk around the inside edge without being pulled off into the empty space in the centre?

I`m guessing the variables are density and spin and diameter , also distance from its sun (maybe) what about a doughnut moon orbiting it .! . would be interesting to know where the lagrange points would be , thanks for my new year headache ! these questions make me more certain I have a doughnut attributes in my head :)
anyway happy new year ! cant believe three years is nearly up ,Kevin hasn't had anyone like him to take his place as a motorcycling journalist . I hope his family are well.

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